Instruments

In the following section, Some instruments used in monitoring of dams are briefly described.

Piezometers measure pore-water pressure and ground water levels. Piezometer measurements help engineers to Monitor water levels, Predict slope stability, Design and build for lateral earth pressures, Design and build for uplift pressures and buoyancy, Monitor seepage and verify models of flow.

Inclinometers may be installed to check that actual movements of a structure correlate to those predicted during the design phase. Inclinometers are installed to monitor the magnitude, direction, and Corrective Measures rate of movement. This information helps engineers determine the need for corrective measures. Inclinometers are installed for long term monitoring to detect Long-Term Performance changes in ground conditions or in the structure itself. Inclinometers, particularly in-place inclinometers that are monitored continuously, can provide early warning of catastrophic failure. Horizontal inclinometers are used to monitor settlement in foundations and embankments. Inclinometers monitor movement, a direct measure of stability, so they are often used in site investigations. Installed at the proposed site for a dam, an inclinometer might detect movement at a sub-surface shear plane. The shear plane could cause problems later when the reservoir behind the dam is filled and pore-water pressure along the shear plane increases.

Beam sensors are used for monitoring settlement, heave, lateral deformation, or convergence. Two version of beam sensors are horizontal version for monitoring settlement and heave and vertical version for monitoring lateral displacement and convergence.

Tiltmeter is used for monitoring changes in inclination. It is used for monitoring the rotation of concrete dams.

Beam sensors differ from tiltmeters in two important respects: First, the beam sensor has a defined gauge length, typically 1 to 3 meters, so changes in tilt can be converted simply and accurately to millimeters of movement (settlement, heave, convergence, or lateral displacement). Second, beam sensors can be linked end-to- end to monitor differential movements and provide absolute displacement and settlement profiles.

Tiltmeters typically have a more limited function, that of monitoring rotation. A tiltmeter can be used with a large number of tilt plates to detect differential movement in a structure, but the resulting data cannot provide absolute displacement and settlement profiles. In general, however, both types of sensors can be used to evaluate the performance of dams under load. They Provide early warning of threatening deformations, allowing time for corrective action to be taken or, if necessary, for safe evacuation of the area.

Borehole Extensometers are used to monitor settlement, heave, convergence, and lateral deformation in soil and rock. Typical applications include monitoring settlement or heave in excavations, foundations, and embankments. Data from the extensometer indicate the depths at which settlement has occurred as well as total amount of settlement.

Pneumatic settlement cells provide a single-point measurement of settlement. They can be read from a central location and are particularly useful where access is difficult. They are used to monitor consolidation in the foundation during construction and monitor long-term settlement in the foundation and fill.

Surface Extensometers (Tape Extensometer) is used to determine changes in the distance between reference points anchored in walls or structures of an excavation, precision measure of convergence.

Load cells are used to proof-test and measure loads in tie- backs, rock bolts, ground anchors, and struts. They are also used for monitoring the performance of anchor systems.

Total Pressure Cells measure the combined pressure of effective stress and pore-water pressure. In general, they are used to verify design assumptions and to warn of soil pressures in excess of those a structure is designed to withstand. Total pressure cells are installed within fills to determine the distribution , magnitude and directions of total stresses. They can also be installed with one surface against a structure to measure total stresses acting on upstream face of the dam, retaining walls, against piles, and pipes. In Embankment Dam Total pressure cells used to confirm design assumptions and An array of cells provides data to determine distribution, size, and direction of total stresses within the clay core.

Soil strainmeters are used for monitoring horizontal strain in embankments and monitoring tension cracks in earth structures.

Embeded jointmeters are used for monitoring movement at joints in mass concrete structures. It is used in mass concrete structures such as dams, abutments, foundations to monitor movement at joints.

Surface-mount jointmeters are used to monitor movement at joints and cracks in concrete structures under harsh environments and submersion. Typical application include monitoring movement at submerged construction joints in concrete dams and monitoring joints or cracks in tunnels and tanks.

Strain gauge is used for monitoring strain in steel structural members, reinforced and mass concrete.

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